So-called locking helical thread systems are well-known; for example, those in which the binding force applied to the fastened parts is increased as tightening torque alters the respective pitch diameters of the male and female elements. U.S. Pat. No. 3,480,170 is of this type. In the cited disclosure, a helical element carried by one part rides on a helical mating element on the other part by means of respective tooth forms which are mutually cammed as the parts are relatively rotated. In this patent, the mutual action causes the pitch diameter of the female element to become effectively smaller. By this interaction, relative axial movement of the two helices is translated into radial pressures and the resistance to inadvertent backing off of one part is increased.
Other examples of threads employing helical locking flanges may be found in U.S. Pat. Nos. 4,125,201 and 3,463,340.